Around 70 million years ago, during the Late Cretaceous, the plant-eating reptiles probably lived in multigenerational herds — just like modern grazers, a dinosaur track site in Alaska suggests.

The tracks, a jumbled bunch of large and small footprints, are the first solid sign that duck-billed dinos, or hadrosaurs, settled together year-round in the polar region, which at the time probably had a climate similar to the modern Pacific Northwest’s. The animals “weren’t just accidental tourists in this area,” says Anthony Fiorillo. “They were thriving.”

People stereotype dinosaurs as swamp creatures, says Fiorillo, a paleontologist at the Perot Museum of Nature and Science in Dallas. But, he says, dinosaurs were actually a highly adaptable bunch that lived in a variety of habitats — even the Arctic.

In 2007, Fiorillo and colleagues discovered thousands of tracks clustered in a soccer field–sized patch of mountain in Denali National Park and Preserve. Measurements of the tracks, three-lobed footprints that dapple the rock like a layer of sunken leaves, suggest that duck-billed dinosaurs ranging from youngsters to adults shared a similar stomping ground, the team reports June 30 in Geology.

Like the African plains roamed by young and old herd animals today, Alaska might have been the “Serengeti of the Cretaceous,” Fiorillo says.